The power semiconductor industry uses various tests to validate and categorize the product worthiness of a particular semiconductor for specific applications. The subject of the present invention is a novel testing system comprising a method and apparatus tailored to test power MOSFET devices in lieu of less reliable tests such as dv/dt immunity tests, leading to improved yields and more reliable validation.
Metal oxide semiconductor field effect transistors (MOSFETs) are used in certain circuit applications as a control switch, where its purpose is primarily to control the drain current via the gate voltage. An overview of the structure of the MOSFET is well known in the art, and many references detail the features of the power MOSFET such as the technical note by Vrej Barkhordarian, which can be found at http://www.irf.com/technical-info/appnotes/mosfet.pdf, the contents of which are fully incorporated herein by reference.
A MOSFET's switching performance is an important feature of the device characterized voltage change rate (dv/dt) across a capacitor. For example, RG can represent the distributed resistance of the gate, and its value is (approximately) inversely proportional to the active area. LS and LD represent source and drain lead inductances, respectively. The values of input (Ci), output (Co) and reverse transfer (Cr) capacitances are used by circuit designers as a starting point in determining circuit component values.
Data sheet capacitances are defined in terms of the equivalent circuit capacitances as:Ci=CGS+CGD, (CDS shorted)Cr=CGDCo=CDS+CGD
The gate-to-drain capacitance, CGD, is a nonlinear function of voltage and is the most important parameter because it provides a feedback loop between the output and the input of the circuit. CGD is also called the Miller capacitance because it causes the total dynamic input capacitance to become greater than the sum of the static capacitances.
Although input capacitance values are useful, they do not provide accurate results when comparing the switching performances of two devices from different manufacturers. Rather, size effects and transconductance hinder such comparisons. To overcome this limitation, it has been found that the gate charge, rather than capacitance, is a more useful parameter from the circuit design point of view.
FIG. 1 shows a typical gate charge waveform and FIG. 2 illustrates a test circuit that could be associated with the graph of FIG. 1. Referring to FIGS. 1 and 2 collectively, when the gate is connected to a supply voltage (i.e., switch S is opened), VGS starts to increase until it reaches a threshold voltage VTH, at which point the drain current Id starts to flow and the CGS starts to charge. The charge associated with this period between t0 and t1 is referred to as QGS1, as shown in FIG. 1. After the threshold voltage is reached at t1, CGS continues to charge and the gate voltage continues to rise as the drain current rises proportionally. At time t2, CGS is completely charged and the drain current reaches the predetermined current ID and stays constant while the drain voltage starts to fall. The charge associated with this portion of the graph between t1 and t2 is QGS2, also shown in FIG. 1. When CGS is fully charged at t2, VGS becomes constant and the drive current starts to charge the Miller capacitance, CDG. This continues until time t3.
Charge time for the Miller capacitance is larger than that for the gate to source capacitance CGS due to the rapidly changing drain voltage between t2 and t3 (current=Cdv/dt). Once both of the capacitances CGS and CGD are fully charged, gate voltage (VGS) starts increasing again until it reaches the supply voltage at time t4. The gate charge QGmin (=QGS+QGD) corresponding to time t3 is the bare minimum charge required to switch the device on. Good circuit design practice dictates the use of a higher gate voltage than the bare minimum required for switching, and therefore the gate charge used in the calculations is QG corresponding to t4. The advantage of using gate charge is that the designer can easily calculate the amount of current required from the drive circuit to switch the device on in a desired length of time because Q=CV and I=Cdv/dt, the Q=Time*current.
Peak diode recovery is an important consideration of MOSFET operation and is defined as the maximum rate of rise of drain-source voltage allowed, i.e., dv/dt capability. If this rate is exceeded, then the voltage across the gate-source terminals may become higher than the threshold voltage of the device, forcing the device into current conduction mode, and under certain conditions a catastrophic failure may occur. There are two possible mechanisms by which a dv/dt induced turn-on may take place. FIG. 3 shows the equivalent circuit model of an N-channel power MOSFET, including a parasitic bipolar power junction transistor (BJT).
The first mechanism of dv/dt induced turn-on becomes active through the feedback action of the gate-drain capacitance, CGD. When a voltage ramp appears across the drain and source terminal of the device, a current I1 flows through the gate resistance, RG, by means of the gate-drain capacitance, CGD. RG represents the total gate resistance in the circuit, and the voltage drop across it is given by:VGS=I1*RG=RG*CGD(dv/dt)
When the gate voltage VGS exceeds the threshold voltage of the device VTH, the device is forced into conduction. The dv/dt capability for this mechanism is thus set by:dv/dt=VTH/(RG*CGD)
From the foregoing, it is clear that devices with a low threshold voltage VTH are more prone to unwanted dv/dt turn-on. Gate circuit impedance must be carefully considered to avoid this phenomenon. The second mechanism for the dv/dt turn-on in power MOSFETs is through the parasitic BJT. The capacitance associated with the depletion region of the body diode extending into the drift region is denoted as CDB and appears between the base of the BJT and the drain of the MOSFET. This capacitance gives rise to a current I2 to flow through the base resistance RB when a voltage ramp appears across the drain-source terminals. With analogy to the first mechanism, the dv/dt capability of this mechanism is:dv/dt=VBE/(RB*CDB)
If the voltage that develops across RB is high enough, then the base-emitter junction is forward-biased and the parasitic BJT is turned on. Under the conditions of high dv/dt and large values of RB, the breakdown voltage of the MOSFET will be limited to that of the open base breakdown voltage of the BJT. If the applied drain voltage is greater than the open-base breakdown voltage, then the MOSFET will enter avalanche mode if the current is not externally limited. Increasing the dv/dt capability therefore requires reducing the base resistance RB by increasing the body region doping and reducing the distance current I2 has to flow laterally before it is collected by the source metallization. As in the first mode, the BJT related dv/dt capability becomes worse at higher temperatures because RB increases and VBE decreases with increasing temperature.
While the data sheets of power MOSFETs include different specifications, most often some very important parameters such as gate resistance RG and gate charge QG are not directly measured despite directly affecting the switching performance of the devices. The increased need for more efficient and robust designs, such as nonisolated synchronous buck power converters and other devices driven by the portable electronic equipment, led manufacture to be keenly interested in having the above identified parameters tested and confirmed before delivery. To meet the needs of the customers, internal gate resistance testers were developed such as the DG900-2 and DG500 by the assignee of the present application, Dolian Graphics, Inc. of Baldwin Park, Calif. Test procedures were integrated into existing testing procedures to form complete testing systems such as the Integrated Rg/Gc Test System by Dolian.
The desired high switching frequencies are limited by the switching losses in the high-side MOSFET. As the MOSFET transitions on and off more rapidly, the switching losses are reduced. However, increasing the on and off transitions result in a more susceptible low-side MOSFET to dv/dt induced turn-on. Dv/dt turn-on occurs when the synchronous MOSFET is active despite the gate-drive signal's command to turn off. In a buck power converter, for example, when the high-side MOSFET is on, the low side must be off. If the dv/dt is such that it turns on the synchronous MOSFET on, a shoot-through current can result as a result of simultaneous conduction of both MOSFETs. As stated above, when the applied dv/dt results in a gate voltage that exceeds the MOSFET gate-to-source threshhold voltage, the converter's reliability and overall efficiency suffers. Thus, one critical parameter for testing the semiconductor is dv/dt immunity. Dv/dt immunity is characteristic of a MOSFET to resist inadvertently turning on in the presence of a high Cdv/dt presence.
Testing for dv/dt immunity is at the forefront of validation through testing in the semiconductor industry. Although much work has gone into improving the dv/dt immunity test, it is still very unreliable and requires high limits or tolerance to be placed on the bands of acceptable components. These high bands or tolerance lead to many viable and operable components being discarded for failing the tolerances limits, and the imprecision of existing testing still allows too high of a percentage of bad components to incorrectly pass the test and be delivered to customers. A bad component is then typically installed into a larger component whose failure is much more costly than replacing the original semiconductor. The inefficiency of the percentage of excluding working components along with falsely passed non-working components has led to the search for more reliable testing of power MOSFETs using existing testing equipment. Further, a difficulty with present dv/dt testing is that it is highly test set-up sensitive, in that results vary widely from one set-up and one tester to another. This disparity leads to inconsistency and higher limits to avoid the false positive passing of a bad component.